The present trend in papermaking is toward the manufacture of sheets with higher brightness and opacity. This type of paper is now made mostly through the use of extenders such as calcined clay and titanium dioxide. These materials present a number of disadvantages, however, in that they are rather expensive and unavailable in sufficient supply to satisfy the needs of large scale paper manufacturers. The competition for the limited quantities available further causes the price of these materials to remain high or to further increase. Consequently, their use is effectively limited to high grade, expensive papers which are usually produced in more limited quantities.
The papermaking industry has been clamoring for the development of a filler-extender material which is inexpensive, available in large quantities and which provides the desired properties in the final paper.
Precipitated calcium carbonate has been used as a filler material in paper for many years. Precipitated calcium carbonate has the advantage of being able to be produced in large quantity at relatively low cost. Therefore, it was decided to develop a form of precipitated calcium carbonate having a morphology which when used as a filler or extender in paper, affords the high level of brightness and opacity in the final paper product comparable to that achieved with the more expensive calcined clay and titanium dioxide filled papers.
One method of altering the morphology of a crystalline substance, such as precipitated calcium carbonate, in order to change its properties, is by Ostwald ripening or heat-aging.
Conventional heat-aging, also known as Ostwald ripening, is a process whereby crystals, such as of calcium carbonate, initially at a higher internal energy state, and having a relatively small average particle size and relatively high phase solubilities, undergo a phase transformation by dissolving and redepositing on crystals at a lower internal energy state. The process results in a final crystal product characterized by greater perfection of its crystal lattice structure, a narrower particle size distribution, greater degree of particle discreteness and lower surface energy.
The procedure for conventional heat-aging of precipitated calcium carbonate produced by the reaction of calcium hydroxide and carbon dioxide is to endpoint the precipitated calcium carbonate synthesis at pH 8.0, screen the material to remove the impurities, and heat a 10% by weight solids slurry to the aging temperature (usually 80.degree. C.). The pH of the system rises to approximately 10.5 due to the unreacted Ca(OH).sub.2 in the slurry. The aging reaction can be monitored by measuring the surface area of the calcium carbonate at hourly intervals.
Unfortunately, conventional heat-aging is a slow, time consuming and highly capital intensive process. Heat-aging of calcium carbonate with an initial morphology having a higher surface area and smaller average particle size to a final morphology having a surface area of from about 3 to about 15 m.sup.2 /g and an average discrete particle size of from about 0.2 to about 0.9 microns typically takes extended periods of time ranging up to several hundred hours, depending in part on the degree of purity of the starting material and the aging temperature. The time required for heat-aging is inversely dependent on the purity of the starting material, the purer the material, the shorter the aging time required. Calcium carbonates containing impurities such as magnesium carbonate, at levels as low as several weight percent, require considerably longer heat-aging time to rearrange their morphologies. The time required for heat-aging is also inversely dependent on the aging temperature; a longer time is required at lower temperatures, and a shorter time is required at higher temperatures. In order to produce commercial scale quantities of product obtained by the conventional heat-aging process, large equipment volumes are needed, thus making this process uneconomical.